A fuel level, such as for a vehicle, is often displayed by means of a needle indicator instrument of a crossed coil type. The associated sensor for detecting the level in the tank is typically provided by a resistor, the resistance of which varies as a function of the level itself. Hereinafter, the term resistance will be used for simplicity to indicate both a resistor and its associated resistance. It is the value of this variable resistance which provides the information for controlling a reserve level lamp. In particular, this lamp is to be lit when the variable resistance of the level sensor is above, or below, a predetermined threshold value.
In known prior art systems, there is typically provided an electronic interface between the indicator instrument and the level sensor for the purpose of damping variations as may be caused by movements of the fuel during motion of the vehicle. In FIG. 1 there is illustrated a system of the type described above. As can be noted, this system basically comprises three modules or units: a level sensor SENS, an electronic processing circuit EL, and an indicator instrument STR. As previously stated, the sensor SENS is of variable resistance type provided by a resistance Rj which is variable as a function of the level of fuel in the tank.
The indicator instrument STR, as expected, is of the crossed coil type, illustrated in the drawing as two resistances R1 and R2. These coils R1 and R2 are connected in series and supplied by the vehicle's supply voltage Val. The currents which flow in the coils R1 and R2 are indicated respectively I1 and I2. The information displayed by the indicator instrument STR, that is, the position assumed by the needle of the instrument, depends on the ratio between the currents It and I2. This ratio depends, in turn, on the current arriving at or leaving the common node between the coils R1 and R2. Originally the sensor SENS was connected directly to this common node so that the ratio between the currents Id and I2 varied upon variation of the resistance Rj.
Currently, the prior art systems use instead an electronic processor circuit EL having an input IN connected to the sensor SENS, and an output OUT connected to the common node of the instrument STR. The processor circuit EL typically comprises a current generator acting to generate a biasing current Ib which is injected into the resistance Rj of the sensor SENS supplied by the supply voltage Val.
This biasing current Ib naturally induces a voltage across the resistance Rj of the sensor SENS. This voltage is sent to a low pass filter LPF the filtering effect of which is to substantially damp oscillations in the value of the resistance Rj due to oscillations of the fuel in the tank. The output of the filter LPF is then connected to an output circuit DR which, in turn, provides the output OUT of the processor circuit EL. The output OUT is operable to control the indicator instrument STR.
A circuit acting to control a reserve lamp, typically associated with the indicator instrument STR, typically also uses a signal generated by the sensor SENS, taken, for example, from within the processor circuit EL. The signal causes illumination of the reserve lamp when the fuel level in the tank falls below a predetermined threshold level.
The characteristics which a reserve lamp control circuit desirably has are as follows. The precision of the control circuit should be independent of the values of the resistance of the coils R1 and R2 of the indicator instrument STR. Otherwise, it would make calibration of the switching point of the lamp necessary in that the resistances R1 and R2 provide a significant source of inaccuracy. The circuit should damp variations in the signal emitted by the sensor SENS due to movements of the fuel. This damping should be similar to that used to provide the level indication in such a way that the variations in the state of the reserve lamp are correctly correlated with the movements of the needle of the indicator instrument STR.
In addition, the circuit should allow for programmability of the switching threshold of the reserve lamp, for example, by means of the selection of a value of a resistance. The circuit should also have a programable hysteresis, for example, by selection of the value of a resistance, to limit as much as possible the illuminations of the reserve lamp caused by successive switchings when the fuel level is close to the threshold level.
FIG. 2 illustrates an alternative prior art form of the system of FIG. 1. In FIG. 2 and in the subsequent drawings, parts and elements already described with reference to the preceding figures have been assigned the same reference numerals and/or letters. In the circuit of FIG. 2, the decision to change the state of the reserve lamp, indicated LAMP, is taken by a comparator CMP on the basis of the measurement of the output resistance, also indicated Rj, with which the output circuit DR controls the indicator instrument STR.
In the drawing for completeness, there have also been shown an output current IL and an output voltage VL of the processor circuit EL. In fact, a voltage divider Rp1, Rp2 on the one hand, and a resistor Rs having output resistance Rj on the other, form a Wheatstone bridge which, based upon the direction in which it is unbalanced, sends to the comparator CMP a differential input voltage which can be positive or negative, and, as a consequence of which, the reserve lamp LAMP is extinguished or illuminated. Since the output resistance of the processor circuit EL is equal to the resistance Rj of the sensor SENS averaged in time by the low pass filter LPF, the change in state of the lamp LAMP is effected on the basis of an average resistance Rj, as well as by the constant resistances Rs, Rp1 and Rp2, according to the following equation: EQU Rj=Rs Rp2/Rp1
In this way the characteristics for controlling the reserve lamp LAMP listed above are satisfied.
The control circuit for the reserve lamp LAMP has a precision which is independent of the indicator instrument STR in that only the processor circuit EL plays a part in generating the output resistance Rj of the output circuit DR. This normally would have to correspond to the average value of the resistance Rj of the sensor SENS from which the switching threshold depends. As far as the correlation is concerned, it is seen that there is a single filter LPF common both to the circuit for controlling the lamp LAMP and the processor circuit EL of the indicator instrument STR, and, therefore, their dynamic behavior is closely correlated.
As far as the switching threshold is concerned, this can be easily programmed by choosing the values of the resistances Rs, Rp1 and Rp2. As far as the hysteresis is concerned, however, the circuit illustrated in FIG. 2 does not provide for it. This can easily be achieved, however, for example by modifying the value of the resistor Rp1 or Rp2 or both of these, on the basis of the state of the lamp LAMP, using controlled switches which add or subtract resistances in series or in parallel. However the prior art arrangement, illustrated in FIG. 2, has some disadvantages. These disadvantages include primarily a lack of precision and in the errors introduced by the circuit for controlling the reserve lamp LAMP, and will be further discussed hereinafter.